Rodent body mass did not decrease by the projected 4% per °C across 9 to 20 North American species, between NEON sites or between years at NEON sites (2016–2024) and at Portal, Arizona (1977–2025)

Small mammals are said to get smaller as climate gets warmer. A recent projection gives a 10-21% decrease in mass for 2.5-5.5 °C of warming, approximately -4% per °C. We tested this rate with live-capture data. We used 36,480 weighed adult captures at 44 NEON sites (2016-2024), taken from NEON Biorepository sample records on GBIF, and 66,730 weighed adult captures of 12 rodent species from the Portal Project, Arizona (1977-2025). For each species we estimated the slope of adult log mass on temperature in three ways: between NEON sites against site mean annual temperature, within NEON sites against the anomaly of the previous 12 months, and between years at Portal against the same anomaly. The models adjust for sex, month and site or plot, and for pregnancy at Portal. The pooled slopes were -0.2% per °C between NEON sites (9 species; 95% CI -1.5 to +1.1), -0.7% within NEON sites (20 species, 42 sites; -1.4 to +0.1), 0.0% with species-year effects (-1.0 to +1.0) and -0.3% at Portal with gridded temperature (-1.9 to +1.3). All pooled CIs exclude -4% per °C. The two widespread species Peromyscus maniculatus (28 sites) and P. leucopus (19 sites) had no Bergmann cline in mass. Some species had significant slopes of 3-9% per °C, in both directions. The Portal slope depended on the temperature series: it was +1.0% (+0.1 to +1.9) with the site station. Adult mass at Portal did not decrease over 1977-2025, although the site warmed. Portal hindfoot length decreased 1% per decade, but most of the decrease was a step after the 2020-2021 trapping gap, the same in all species, which suggests a measurement change. Limitations: NEON covers only 9 years with small anomalies, the NEON captures are those that gave a sample, Portal is one site, and multidecadal evolutionary change was not tested. The data do not support a general short-term or space-for-time shrinkage of about 4% per °C in these rodents.

body size · Bergmann's rule · climate warming · small mammals · rodents · NEON · Portal Project · space-for-time substitution · GBIF · Peromyscus